The market demand for solar energy collection systems in the form of photovoltaic (PV) cells is growing in excess of 25% per year globally due to factors including higher oil prices and government policies addressing such environmental issues as global warming. The dominant substrate material for PV is silicon, which accounts for about 90% of installed commercial units at the present time. A serious shortcoming in the silicon-based PV value chain, however, is that there is a loss of around 40-50% of the silicon during the wafer cutting process. This situation also exists in the interconnected microelectronics (ME) silicon value chain.
The current process for developing a PV cell is a multi-step chain of value-added activities, transforming basic silicon into a power-generating device. With each step, silicon is refined and shaped to enable placement into a solar cell. However, this value chain is not without inefficiencies. During the critical step where silicon ingots are sawed into thin wafers, roughly 40% of the original ingot ends up as spent (or waste) kerf slurry resulting from the most prevalent steel-wire-saw technology using SiC powder in polyethylene glycol (PEG 200).
The spent slurry product from the wafer cutting process generally consists of very fine solid particles within a liquid phase. The solid particles are irregular shaped and consist mostly of silicon carbide of between 15-20 micrometers effective diameter. The remaining particles are from the steel wire saw and silicon wafer. The steel particles may be associated with the silicon carbide particles and are generally less than 2-4 micrometers in effective diameter. The silicon particles arc generally free of silicon carbide and in the particle size range of 1-2 micrometers. During the wire sawing operation the silicon carbide starting material is slightly abraded arid smaller particles in the range of 5-10 micrometers are formed over time.
Therefore, while a raw material silicon shortage exists today for the PV industry that is driving prices toward the electronic-grade silicon (EG-Si) level, about half of all silicon produced for the ME and PV industries is being landfilled.
Although the silicon particles lost during this step are of the same purity as the original ingot, there exist no commercially viable technologies to recover and reuse this silicon. The main reason for this state of art is that the spent slurry can be a very complex, colloidal mixture of extremely small particles in the range of 0.1 to 30 μm—with the silicon portion being less than about 2-5 μm in effective diameter (comparable to the size of bacteria). Efforts to physically separate these silicon particles from the mixture are severely hampered by wire-saw particle impurities (mostly iron, copper, and zinc) that prevent the attainment of the original ingot purity. Even if it were possible to completely remove the wire-saw particles from the slurry by physical means, the remaining ultrafine silicon powder is both dangerous to handle (due to potential dust explosions) and extremely difficult to melt using conventional furnace technology.
The effect of this market need on the overall economics of the PV industry is significant. It has been well-documented that the solar industry has suffered from a major silicon feedstock shortage since 2005.1 During these past 4 years, more than 40% of the >100,000 tonnes of silicon produced during this timeframe was discarded due to the inability to recycle polysilicon. This inefficient use of a critical PV cell building block resulted in a cumulative economic loss to the solar industry of at least $2 Billion over the period 2005-2008.2 Moreover, given that the cost of silicon feedstock comprises almost 20% of a PV cell's total cost3, discarding approximately 40% of the feedstock has been an important contributor to the economics preventing grid-parity and broader adoption of PV cells. 1Travis Bradford, “Polysilicon: Supply, Demand & Implications for the PV Industry,” Greentech In Detail, (Jun. 25 2008) [Prometheus Institute], Pg. 24.2During 2005-2008 period, average polysilicon production for PV was 25K tonnes.'yr., and avenge contract price was $50/kg.3Bradford, “Polysilicon: Supply, Demand & Implications for the PV Industry,” Greentech In Detail, (Jun. 25, 2008) [Prometheus Institute], Pg. 29.
Therefore, there is a need to recover silicon in a form and purity suitable for reuse within the silicon-wafer based PV industry. U.S. Provisional Patent Application No. 61/044,342, filed Apr. 11, 2008, incorporated herein by reference in its entirety, describes a multistep process for recovering silicon granules from spent wafer-sawing operations. The process described therein can include a 2-stage iodine-catalysed reaction sequence that can operate at temperatures between 800-1300° C. to produce a granular silicon product. The purity of silicon recovered can reach 99.9999 wt % (i.e., 6 nines or 6N) and possibly higher levels under certain operating conditions.
However, for the highest efficiency PV cells in use today it may be preferable to utilize a higher-purity silicon. For instance, it may be desirable to obtain a silicon purity of 8N (i.e., 99.999999 wt %).
Therefore, there remains a aced in the art for commercial operations that can efficiently separate the silicon particles from the remainder of the slurry mixture. Furthermore, there exists a need for ways of converting these fine silicon particles into a useable form for application in the commercial production of semiconductor devices such as photovoltaic solar cells. Also, there remains a need in the an for commercial operations that can recover and/or purify silicon to increased purities (e.g., 8N) from various sources, such as the spent wafer-sawing slurry produced in the PV and ME industries.